The RB69 DNA polymerase binding-pocket residue Tyr567 was shown previously to be a key determinant of fidelity because the replacement Y567A causes a powerful mutator activity. Ser565 is a nearby component of the binding pocket. Unlike Y567A, the replacement S565G has only a small impact on fidelity. However, combining Y567A and S565G reverses most of the mutator activity of Y567A. This is a unique and instructive observation in structural studies of polymerase fidelity, and we are working through a detailed examination of the mutational propensities of these constructs both in vivo and in vitro.